Treating solution and treating method for forming protective coating films on metals

ABSTRACT

A metallic surface treating solution characterized in that it is an aqueous solution at pH 0.1 to 6.5 comprising a source of at least one selected from the group consisting of Mo, W, V, Nb, Ta, Ti, Zr, Ce, Sr, and trivalent chromium, an oxidizing substance source, and an oxyacid or oxyacid salt of phosphorus or its anhydride, a surface treating method using the treating solution, and metals thereby treated on the surface.

BACKGROUND OF THE INVENTION

This invention generally relates to a surface treating solution forzinc, copper, nickel, silver, iron, cadmium, aluminum, magnesium, andtheir alloys, a method of applying surface coatings, and coated metallicmaterials. The invention specifically relates to a surface treatingsolution and a treating method for forming protective coating films onzinc- and zinc alloy-coated iron parts, and surface treated metallicmaterials.

There are various films as protective coating films on zinc, copper,nickel, silver, iron, cadmium, aluminum, magnesium, and their alloys.However, no such film that corresponds to any according to the presentinvention has been found yet, and this invention provides newlydiscovered coating films. The most common of corrosion-preventivemethods in use for iron articles and parts is coating with zinc or zincalloy (hereinafter called “galvanizing”). Galvanized iron articles andpins, if used as they are, would readily form a zinc white rust. Toavoid this, they are usually provided with a protective coating filmover the galvanized surface. Protective coating films that areconventionally used on zinc coat are formed by phosphate and chromatetreatments. Chromate treatment is divided into three types;electrolytic, coating, and reaction type chromate treatments. Thesetreatments are applicable not only to zinc but also to aluminum,cadmium, magnesium, and their alloys.

Phosphate treatment is a process, as taught in Patent Application KokaiNo. 3-107469, which comprises immersing an object to be coated in atreating solution which consists essentially of zinc ion and phosphateion as film-forming components and fluoride ion or complex fluoride ionas an etching or film-densifying agent, heated to 40 to 50° C. or up toabout 75° C., thereby forming a coating film on the object, waterwashing, and then drying the coated object. The surface of the coatingfilm thus obtained is very rough with the needle crystals of zincphosphate piled up. This surface condition helps improve the adhesion ofpaint and enhance the corrosion resistance of the painted surface,achieving the dual purpose of the film. However, the film beforepainting is seriously short of rust-inhibiting capacity (corrosionresistance). Moreover, the surface as treated looks dull gray to grayishwhite and lacks ornamental effect. Since the treated surface is notaesthetically attractive, it is not suited for articles that are partlyor wholly unpainted. Phosphate films essentially contain fluoride ion orcomplex fluoride ion without which they cannot be formed, but either ionis strongly corrosive and comes in the list of substances under emissioncontrol. High treating temperature, and extra equipment and cost forheating are additional disadvantages.

On the other hand, chromate film before painting is superior tophosphate film in corrosion resistance. However, chromate treatment hasrecently caused growing concern, because of the adverse effects upon thehuman beings and the environments of the treating solution thatnecessarily uses poisonous hexavalent chromium and also because of thechromium itself that dissolves out of the treated articles. This is aninsurmountable problem since chromate film essentially depends on thehexavalent chromium for its corrosion resistance. Another knotty problemthat is always associated with electrolytic chromate treatment in whicha chromate film is formed by electrolysis is the problem of throwingpower, especially with workpieces of components naturally of farintricate configurations than steel sheets. In addition, the mist ofchromic acid that results from the electrolysis can cause more seriousenvironmental pollution than other known processes. Coating typechromate treatment comprises applying an acidic aqueous solutionessentially containing chromic acid to a metallic surface and withoutwater washing, drying the coated surface with heat. Like electrolyticchromating, the coating type is not suited for workpieces of complexconfigurations. Moreover, the process has its limitation on theuniformity of coating film thickness. This combines with the omission ofwater washing to make the treated surface as uneven as with thephosphate film. The coated film, therefore, is unable to satisfy theusers' aesthetic requirements when used alone and, like the phosphatefilm, it is commonly employed as a mere undercoat. Reaction typechromate treatment, by contrast, is often adopted as finish coating aswell as undercoating because of the uniform appearance and stablecorrosion resistance of the coating film. It has the unsettled pollutionproblem of hexavalent chromium, however.

The present invention has for its object to form protective coatingfilms which combines a uniform, good appearance and corrosion resistanceon the surfaces of zinc, copper nickel, silver, iron, cadmium, aluminum,magnesium, and their alloys, without using noxious hexavalent chromiumor strongly corrosive fluorine compounds. A particularly importantobject is to provide protective coating films or galvanized ironarticles other than steel sheets, for which coating type treatment on anindustrial scale has hitherto been practically difficult.

SUMMARY OF THE INVENTION

With a view to solve the problems of the prior art, the presentinventors have concentrated their efforts and have now successfullyobtained coating films that apparently do not belong to the ordinarycategory of phosphate films or chromate films. It has now been foundpossible to produce coating films having beautiful, bright appearanceand outstanding corrosion resistance, without using hexavalent chromium,by a method which comprises forming a film or a metallic surface eitherby immersion in or electrolysis with a treating solution characterizedin that it is an aqueous solution at pH 0.1 to 6.5 comprising a sourceof at least one selected from the group consisting of Mo, W, V, Zr, Sr,Nb, Ta, Ti, Ce, and trivalent chromium, an oxyacid or oxyacid salt ofphosphorus or an anhydride thereof, and an oxidizing substance source,water washing, and drying. It has also been found that protectivecoating films with enhanced corrosion resistance can be obtained bywater washing a film formed by immersion or electrolysis and, withoutdrying bringing the washed film into contact with a resin or inorganiccolloid. The coating films obtained in accordance with the inventionhave been found to exhibit great high temperature corrosion resistance,thus solving a problem common with ordinary chromate films; weakenedcorrosion resistance upon heat treatment. It is another feature of theinventive method that, when the treatment is performed by immersion, anexisting equipment for reaction type chromate treatment can be utilizedto an economic advantage.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is an electron micrograph showing the surface texture of acoating film formed in Example 1 of the present invention; and

FIG. 2 is an electron micrograph showing the surface texture of acoating film formed in Example 3 of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail. The treatingsolution according to the invention is an aqueous solution at pH 0.1 to6.5 comprising a source of a metallic cation, oxymetallic anion or thelike of Mo, W, V, Nb, Ta, Ti, Zr, Sr, Ce, or trivalent chromium,oxidizing substance selected from the group consisting of peroxide,hydrochloric acid, hydrobromic acid, nitric acid, and salts thereofsource, an oxyacid or oxyacid salt of phosphorus or its anhydride, andan oxidizing substance source. Although the exact behavior of eachcomponent is unknown, a source of any of various metals such asmolybdate ion, tungstate ion, vanadate ion, niobate ion, tantalate ion,or trivalent chromium ion and an oxyacid or oxyacid salt of phosphorusor its anhydride are presumed to be components that form the skeleton ofa coating film. An oxidizing substance presumably inhibits theionization in a solution of the oxyacid or oxyacid salt of phosphorus orits anhydride and ensures the stability of the solution while, at thesame time, properly etching the metal and promoting smooth filmformation. Suitable oxidizing substance sources include peroxides,chloric acid, bromic acid, nitric acid, and salts thereof.

In the case of alloy substrates that have particularly strongpossibilities of hampering uniform coating film formation, the absenceof an oxidizing substance often makes the film unable to exhibitsatisfactory performance. This can cause phenomena such as the inabilityof forming a thick film due to difficulty of etching or of forming auniform appearance owing to uneven etching and the consequent failure ofobtaining a levelled film surface, and localized chemical synthesis forfilm formation in certain areas and no film formation in the remainder.The presence of an oxidizing substance that controls these phenomenavaries in performance, five to more than ten times, depending on itsproportion to the composition of the treating solution, and therefore aproper amount of such a substance must be used.

The total amount of the metal source, such as molybdate ion, tungstateion, vanadate ion, niobate ion, tantalate ion, or trivalent chromiumion, ranges from 0.2 to 300 g/l, preferably from 0.5 to 80 g/l. If theamount is less than the range, a good film is difficult or impossible toobtain. If any, a too thin film is formed to attain desired performance.If the amount is more than the range, marred film appearance andbrightness and/or a material economic loss due to excessive dipping outcan result. The source is not specially limited, while ammoniumvanadate, sodium tungstate, chromium acetate, and chromium nitrate arecited as examples.

The amount of the oxyacid or oxyacid salt of phosphorus or its anhydrideto be contained should be from 0.2 to 200 g/l, preferably from 3 to 90g/l. If the amount is below the range, it is difficult or impossible toobtain a good film, or a too thin film is formed to attain desiredperformance. If the amount is over the range, the film appearance andbrightness are marred and/or the economic loss due to excessive dippingout can increase materially.

As for an oxyacid of phosphorus, not only orthophosphoric acid but alsohypophosphorous, pyrophosphoric, tripolyphosphoric, and perphosphoricacids and the like can be used. If such an oxyacid is used in the formof a metallic salt, both a metal and an oxidizing substance can besupplied. The amount to be contained is between 0.2 and 400 g/l,preferably between 2 and 100 g/l. An insufficient amount would make theresulting solution or the film-forming rate instable, but an excessiveamount would cause much economic loss due to wasteful dipping out. Itwould sometimes happen in either case that no coating film is formed.

A pH from 0.1 to 6.5 is desired, a narrower range from 1.0 to 4.0 beingpreferred. If the pH is too low a uniform film is difficult to obtain,but if it is too high, the corrosion resistance tends to decrease tosome extent. Chemicals to be used for pH adjustment are not speciallylimited, usually nitric or sulfuric acid or the like being used when thepH is too high or an alkali such as ammonia or sodium hydroxide beingadded when it is too low.

There is no special limitation to the treatment conditions for theformation of coating film by immersion. The treatment may be conductedunder a broad range of conditions, e.g., the conditions for ordinaryreaction type chromate treatment (bath temperature=20-30° C.; treatingtime=20-60 sec.; with stirring) or such conditions that treatingtime=250 sec., without stirring. The conditions for film formation byelectrolysis are: current density=up to 30 A/dm², preferably 0.5-3A/dm²; duration of current flow=1-1200 sec., preferably 30-180 sec. Evenwith a lower current density a film is formed, but under the inventionthe film formation not necessarily depends on electrolysis, and whethera film has been formed by electrolysis or by reaction is hardlydiscernible. Hence it is impossible to set the lower limit to thecurrent density. When the density is too high, a surface defect known as“burn” or “scorch” develops in the portion subjected to the excessivecurrent density. When the treating time is too short, a film is notformed or, if any, the film is too thin and inferior in corrosionresistance. When the treating time is too long, a dull surface defectsometimes results. Also, the excessive treatment seriously reduces theproductivity.

After a coating film has been formed in the manner described above, thefilm is washed with water. The washing removes surplus matter to providea uniform surface. Unlike phosphate film and coated chromate film, thefilm according to the invention has a uniform, bright appearance. Meredrying after the water washing affords the film the appearance andcorrosion resistance that satisfy user requirements. Where highercorrosion resistance is a necessity, the film formed by the treatment ofthe invention may be painted or additionally coated as desired.Conventionally, chromate treatment or phosphate film treatment has beenused to form a prime coat for painting. Either treatment ends withdrying as the final step. If the surface yet to be dried is painted orotherwise treated, a sound composite film will not result. Under theinvention, by contrast, it has been found possible to paint or otherwisecoat the film formed by immersion or electrolysis and water washed,without being dried up. This is remarkably effective for the improvementin productivity, because, for one thing, it eliminates the expenses andlabor required for the prime coat line (drying step) and for theconveyance of workpieces between painting and coating lines that areotherwise required for conventional processes and, for the other, thereis no need of waiting for the temperature drop of the treated surfacethat has been made hot by drying.

The treating solution may further contain one or two or more substanceschosen from among alkaline earth metals, inorganic colloids, silanecoupling agents, and organic carboxylic acids.

Usable as inorganic colloids are silica sol, alumina sol, titania sol,zirconia sol, and the like, and as silane coupling agents arevinyltriethoxysilane, γg/l y-methacryloxypropyltrimethoxysilane, and thelike.

Although it is rather unthinkable that an alkaline earth metal shouldprecipitate in a coating film, the fact that its addition improves thecorrosion resistance implies its effectiveness in densifying the filmstructure.

The addition of an inorganic colloid, silane coupling agent and the likeis not always warranted for cost and other reasons. However, suchsubstances improve the adhesion of the film when it is to be painted orotherwise coated after the treatment of the invention, thus enhancingthe corrosion resistance of the finished surface.

The use of an acidic aqueous solution as defined by the inventionrenders it possible to form an insoluble, solid film over a zinc surfacewithout the aid of noxious hexavalent chromium or highly corrosivefluoride, sometimes using the same equipment, conditions, and method fortreatment as the conventional reaction type chromate treatment. Thishelps solve the health problems including the concern of general usersabout the escape of hexavalent chromium from ordinarily treatedmaterials, the concern of personnel engaged in the production ofchromate and treatment with it and who have been exposed to noxiouschromic acid, and the environmental concern about the adverse effectsupon wildlife.

The method of the invention is similar to two known methods, chromatetreatment and phosphate treatment. However, it does not seem to fallunder either category when diversified factors, e.g., the composition ofthe solution, appearance of the treated surface, anti-corrosionmechanism, and treatment conditions, are taken into consideration.Chromate treatment is a generic term of treatment procedures using anaqueous solution that contains hexavalent chromium, typified by chromicacid. The coating film thereby formed depends on its hexavalent chromiumcontent for its corrosion resistance. Considering this definition, themethod of the invention that does not use hexavalent chromium is not achromate treatment. Since the resulting film does not contain hexavalentchromium, its anti-corrosion mechanism is not dependent upon thehexavalent chromium content in the film, and hence the film is not achromate one. As a chromate free from hexavalent chromium, trivalentchromate is described in Products Finishing, 52 [9], 71 (1988). Thecorrosion resistance of the coating film so obtained lasts, in a saltspray test, at most 35 to 40 hours (until 5% zinc white rust is formed).Thus the corrosion resistance of an ordinary trivalent chromate film isonly about one quarter to one-fifth that according to the presentinvention. It is presumed that a trivalent chromate film (film structureor anti-corrosion mechanism), like a conventional hexavalentchromium-containing chromate film, depends on the hexavalent chromiumion concentration in the film for its corrosion resistance, and that iswhy the film attains such low corrosion resistance. The facts presentedabove indicate that the film according to this invention differs fromconventional chromate films in anti-corrosion mechanism and that themethod of the invention is not a chromate treatment.

Phosphate treatment on zinc, as described in above-mentioned PatentApplication Kokai No. 3-107469, is a treatment which comprises immersinga workpiece into a treating solution which consists essentially of zincion and phosphate ion as film-forming components and fluoride ion orcomplex fluoride ion as an etching agent (chemical synthesis reactioninitiator) or film-densifying agent and heated to 40˜50° C. or up to thevicinity of 75° C., thereby forming a coating film on the workpiece,water washing, and drying the coated workpiece. The treatment of thepresent invention differs from the phosphate treatment in thecomposition of the solution and in the treating method. In respect ofthe composition the solution of the invention is utterly different inthat it does not require zinc as a film-forming element and fluoride ionor complex fluoride ion as an etching agent. Without these components aphosphate film would not be formed. Also, compared with the phosphatetreatment that requires heating to 40˜75° C. for film formation, thepresent invention can carry out the treatment at ordinary temperatures(20˜25° C.). Thus the two differ in treatment condition too. Acomparison in performance shows that a phosphate film looks grayishwhite and possesses corrosion resistance of not more than 24 hoursbefore it forms zinc white rust in a salt spray test, whereas the filmof the invention is uniform and bright in appearance and exhibitscorrosion resistance of more than 120 hours before zinc white rustingstarts in a salt spray test. Phosphate coating treatment is usuallyfollowed, for added corrosion resistance, by immersion into a diluteaqueous solution of chromic acid, a treatment known as sealing orafter-treatment. Even after this additional treatment, the coating filmretains corrosion resistance for less than 24 hours, before zinc whiterust is formed.

It should be clear from electron micrographs of coating films formed inaccordance with the invention in FIG. 1 (Example 1) and FIG. 2 (Example3) that the films are dissimilar to phosphate films. Compared with aphosphate film that is covered completely with needle crystals [JITSUMUHYOMEN GIJUTSU (Practical Surface Technologies), Vol. 35, No. 1, p. 23,Photo 2 (1988)], the films of the invention show no discernible crystalon the surface.

As described above, the treatment according to the present invention isentirely different from conventional phosphate or chromate coating filmtreatment, when they are compared and studied in diversified aspectsincluding the bath composition, anti-corrosion mechanism, surfaceconfigurations, treating conditions, and appearance of the treatedsurfaces.

The invention is illustrated by the following examples. Tests wereconducted with test specimens that had been properly pretreated withdegreasing, dip in nitric acid, etc., in the following way. Evaluationsof the results were made with regard to the appearance and corrosionresistance and summarized in Table 1.

EXAMPLE 1

A galvanized iron piece (measuring 50×100×1 mm) was coated with a filmby immersion for 90 seconds in a treating solution which was an aqueoussolution containing 18 g chromium nitrate, 20 g 75% phosphoric acid, and15 g 67.5% nitric acid, all per liter, and adjusted to pH 1.8 withammonia. The coated piece was water washed and dried as a test specimen.

Its appearance was visually examined and its corrosion resistance wasevaluated from the result of a salt spray test (JIS Z 2371) conductedfor 120 hours.

EXAMPLE 2

A test specimen obtained by the procedure of Example 1 was heat treatedat 200° C. for one hour to provide a test specimen.

Its appearance was visually inspected and its corrosion resistance wasevaluated from the result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 3

A galvanized iron piece (50×100×1 mm) was coated with a film byimmersion for one minute in a treating solution which was an aqueoussolution containing 5 g ammonium tungstate, 15 g chromium nitrate, 25 g75% phosphoric acid, and 25 g 60% nitric acid, all per liter, andadjusted to pH 2.0 with ammonia. The coated piece was water washed anddried as a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 4

A galvanized iron piece (50×100×1 mm) was coated with a film byimmersion for two minutes in a treating solution which was an aqueoussolution containing 15 g sodium molybdate, 25 g phosphorous acid, and 25g 60% nitric acid, all per liter, and adjusted to pH 2.0 with ammonia.The coated piece was water washed and dried, and then immersed in andcoated with “Kosmer No. 9001” (made by Kansai Paint Co.) as a testspecimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 5

A galvanized iron piece (50×100×1 mm) was coated with a film byimmersion for two minutes in a treating solution of pH 1.0 whichcontained 15 g chromium nitrate, 2 g ammonium vanadate, 25 ghypophosphorous acid, and 18 g 60% nitric acid, all per liter. Thecoated piece was water washed and dried, and then immersed in and coatedwith “Kosmer No. 9001” (of Kansai Paint Co.) as a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 6

A galvanized iron piece (50×100×1 mm) was coated with a film by cathodicelectrolysis for two minutes at a current density of 1 A/dm² in atreating solution which was an aqueous solution containing 10 g ammoniumvanadate, 20 g chromium nitrate, 25 g 75% phosphoric acid, 20 g 62.5%nitric acid, and 20 g colloidal silica, all per liter, and adjusted topH 2.0 with ammonia. The coated piece was water washed and, withoutdrying, immersed in and coated with “Kosmer No. 9001” (of Kansai PaintCo.) as a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 7

A galvanized iron piece (50×100×1 mm) was coated with a film by cathodicelectrolysis for two minutes at a current density of 1 A/dm² in atreating solution which was an aqueous solution containing 5 g ammoniummolybdate, 20 g chromium nitrate, 30 g phosphorous acid, 20 g 62.5%nitric acid, and 20 g colloidal silica, all per liter, and adjusted topH 2.0 with ammonia. The coated piece was water washed and, withoutdrying, immersed in and coated with “Kosmer No. 9001” (of Kansai PaintCo.) as a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 8

A galvanized iron piece (50×100×1 mm) was treated with an aqueoussolution of pH 2.5 which contained 8 g 62% nitric acid, 20 g chromiumnitrate, and 25 g pyrophosphoric acid, all per liter, at a bathtemperature of 30° C. for 80 seconds. The treated piece was immersed inan aqueous solution of colloidal silica to provide a test specimen. Theappearance of the specimen was visually examined and its corrosionresistance was evaluated from the result of a 120-hour salt spray test(JIS Z 2371).

EXAMPLE 9

An aluminum alloy (A1050) piece (50×100×1 mm) was coated with a film byimmersion for 90 seconds in a treating solution which was an aqueoussolution containing 27 g chromium nitrate, 30 g 75% phosphoric acid, and25 g 67.5% nitric acid, all per liter, and adjusted to pH 1.8 withsodium hydroxide, and water washed and dried as a test specimen.

Its appearance was visually inspected and its corrosion resistance wasevaluated from the result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 10

A magnesium alloy (MP1) piece (50×100×1 mm) was coated with a film byimmersion for two minutes in a treating solution which was an aqueoussolution containing 18 g sodium molybdate, 38 g phosphorous acid, and 45g 60% nitric acid, all per liter, and adjusted to pH 2.0 with sodiumhydroxide. The coated piece was water washed, dried, and immersed in andcoated with “Kosmer No. 9001” (of Kansai Paint Co.) as a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 11

An iron piece coated with zinc containing 0.01% iron (50×100×1 mm) wascoated with a film by immersion for 90 seconds in a treating solutionwhich was an aqueous solution containing 18 g chromium nitrate, 20 g 75%phosphoric acid, and 15 g 67.5% nitric acid, all per liter, and adjustedto pH 1.8 with ammonia. The coated piece was water washed and dried as atest specimen.

Its appearance was visually inspected and its corrosion resistance wasevaluated from the result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 12

An iron piece coated with zinc containing 200 ppm iron (50×100×1 mm) wascoated with a film by immersion for one minute in a treating solutionwhich was an aqueous solution containing 5 g ammonium tungstate, 15 gchromium nitrate, 25 g 75% phosphoric acid, and 25 g 60% nitric acid,all per liter, and adjusted to pH 2.0 with ammonia. The coated piece waswater washed and dried as a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 120-hour salt spray test (JIS Z 2371).

EXAMPLE 13

An iron piece coated with zinc containing 5000 ppm iron (50×100×1 mm)was coated with a film by immersion for two minutes in a treatingsolution which was an aqueous solution containing 15 g sodium molybdate,6 g chromium sulfate, 25 g phosphorous acid, and 25 g 60% nitric acid,all per liter, and adjusted to pH 2.0 with ammonia. The coated piece waswater washed and dried and then immersed in and coated with “Kosmer No.9001” (of Kansai Paint Co.) to provide a test specimen.

Its appearance was visually evaluated and its corrosion resistance fromthe result of a 600-hour salt spray test (JIS Z 2371).

COMPARATIVE EXAMPLE 1

A galvanized iron piece with untreated surface (50×100×1 mm) was used asa least specimen, and the time it took until zinc white rust was formedin a salt spray test (JIS Z 2371) was measured.

COMPARATIVE EXAMPLE 2

A galvanized iron piece (50×100×1 mm) was coated with a film byimmersion for one minute in a commercially available trivalent chromatetreating solution (“Aidip Z-348” of Aiko Chemical Co.), water washed anddried as a lest specimen.

Its appearance was visually evaluated, and its corrosion resistance wasdetermined by measuring the time it took for the formation of zinc whiterust in a salt spray test (JIS Z 2371).

COMPARATIVE EXAMPLE 3

A galvanized iron piece (50×100×1 mm) was conditioned on the surfacewith “Preparen Z” (of Nibon Parkerizing Co.) and was coated with a filmby immersion for 15 seconds in a commercially available phosphate filmtreating solution (“Parbond 3300” of Nihon Parkerizing Co.) heated at70° C. The coated piece was aftertreated with “Parlen 1” (of NihonParkerizing Co.) and dried as a test specimen.

Its appearance was visually inspected and the time it took for zincwhite rusting-in a salt spray test (JIS Z 2371) was measured.

COMPARATIVE EXAMPLE 4

The same test specimen as used in Example 9 was immersed in an organiccoating agent “5G018” (of Nihon Hyomen Kagaku) to serve as a testspecimen.

Its appearance was visually examined and its corrosion resistance wasevaluated in terms of the time required for the starting of zinc whiterusting in a salt spray test (JIS Z 2371).

COMPARATIVE EXAMPLE 5

The same test specimen as used in Example 9 was immersed in an aqueoussolution of a water-soluble resin “Cymel UFR” (of Mitsui Cytec) toprovide a test specimen.

Its appearance was visually evaluated and, as for its corrosionresistance, the time required for zinc white rusting in a salt spraytest (JIS Z 2371) was measured.

COMPARATIVE EXAMPLE 6

The same test specimen as used in Example 10 was immersed in an aqueoussolution of a water-soluble resin “Cymel UFR” (of Mitsui Cytec) to serveas a test specimen.

Its appearance was visually evaluated and its corrosion resistance wasdetermined by measuring the time required for zinc white rusting in asalt spray test (JIS Z 2371).

COMPARATIVE EXAMPLE 7

An iron piece coated with zinc containing 3500 ppm iron (50×100×1 mm)was treated with an aqueous solution of pH 1.2 which contained 30 gchromium phosphate and 20 g phosphoric acid, both per liter, for twominutes to form a coating film. The coated piece was water washed anddried as a test specimen.

Its appearance was visually examined and its corrosion resistance wasdetermined in terms of the time required for zinc while rusting in asalt spray test (JIS Z 2371).

COMPARATIVE EXAMPLE 8

An iron piece coated with zinc containing 6500 ppm ion (50×100×1 mm) wascoated with a film by treatment for two minutes with an aqueous solutionof pH 1.2 which contained 25 g chromium acetate and 15 g phosphoricacid, both per liter. The coated piece was water washed and immersed inan aqueous solution containing 10% sodium silicate at 30° C. for 70seconds to provide a test specimen.

Its appearance was visually inspected and its corrosion resistance wasdetermined as the time required for zinc while rusting in a salt spraytest (JIS Z 2371).

The evaluation results of the foregoing examples weft as follows.

TABLE 1 Example Appearance Corrosion resistance Ex 1 Uniform & bright Nozinc white rust in 120 hours 2 Uniform & bright No zinc white rust in120 hours 3 Uniform & bright No zinc white rust in 120 hours 4 Uniform &bright No zinc white rust in 120 hours 5 Uniform & bright No zinc whiterust in 120 hours 6 Uniform & bright No zinc white rust in 120 hours 7Uniform & bright No zinc white rust in 120 hours 8 Uniform & bright Nozinc white rust in 120 hours 9 Uniform & bright 5% zinc white rust in 72hours 10  Uniform & bright No zinc white rust in 120 hours 11  Uniform &bright No zinc white rust in 120 hours 12  Uniform & bright No zincwhite rust in 120 hours 13  Uniform & bright No zinc white rust in 600hours Comp 1 — Entire zinc white rust within 1 hour 2 Uniform & brightZinc white rust within 24 hours 3 Gray˜Grayish white Zinc white rustwithin 24 hours 4 Not uniform Zinc white rust within 24 hours 5 — Zincwhite rust within 12 hours 6 — Zinc white rust within 12 hours 7 Notuniform Zinc white rust within 24 hours 8 Not uniform Zinc white rustwithin 60 hours

As can be seen from Table 1, the surfaces treated with the treatingsolutions according to the present invention exhibited excellentcorrosion resistance and uniform brightness.

In forming a protective coating film on the surface of Zn, Ni, Cu, Ag,Fe, Cd, Al, Mg, or their alloy, the present invention permits theformation of a film which combines uniform, good appearance withcorrosion resistance, without using any noxious hexavalent chromium orhighly corrosive fluorine compound. In particular, the invention makesit possible to form protective films on galvanized iron articles otherthan steels, which have hitherto been practically difficult to protectby a coating type treatment on an industrial scale.

What is claimed is:
 1. A protectively coated metal substrate of Cu, Ag,Fe, Cd, Al, Mg, and alloy thereof, Zn, Ni, or Zn-Fe alloy, wherein themetal substrate is coated with a protective film which is a reactionproduct on the metal substrate of a solution substantially free offluoride ions and containing (i) a source of metallic cations of atleast one metal element selected from the group consisting of Mo, W, V,Ta, Ti, Zr, Ce, Sr and trivalent chromium, (ii) at least one oxyacid ofphosphorus, oxyacid salt of phosphorus, or anhydride of an oxyacid ofphosphorus, said oxyacid being selected from the group consisting oforthophosphoric acid, hypophosphorous acid, pyrophoshorous acid,tripolyphosphoric acid and perphosphoric acid, and (iii) at least oneoxidizing substance selected from the group consisting of peroxide,hydrochloric chloric acid, hydrobromic bromic acid, nitric acid, andsalts thereof.
 2. The protectively coated metal substrate of claim 1,wherein the protective film is overcoated with an organic, inorganic, orcomposite corrosion-preventive coating film.
 3. The protectively coatedmetal substrate of claim 1, wherein said solution further contains atleast one substance selected from the group consisting of alkaline earthmetals, an inorganic colloid, and silane coupling agents.
 4. Theprotectively coated metal substrate of claim 2, wherein the inorganiccolloid is selected from the group consisting of silica sol, aluminasol, titania sol, and zirconia sol.
 5. The protectively coated metalsubstrate of claim 1, wherein said solution is substantially free ofhexavalent chromium ions.
 6. A protectively coated metal substrate ofZn, Ni, Cu, Ag, Fe, Cd, Al, Mg, or an alloy thereof, wherein the metalsubstrate is coated with a protective film which is a reaction producton the metal substrate of a solution substantially free of fluoride ionsand containing (i) a source of metallic cations of at least one metalelement selected from the group consisting of Mo, W, V, Ta, Ti, Zr, Ce,Sr and trivalent chromium, (ii) at least one oxyacid of phosphorus,oxyacid salt of phosphorus, or anhydride of an oxyacid of phosphorus,said oxyacid being selected from the group consisting of orthophosphoricacid, hypophosphorous acid, pyrophoshorous acid, tripolyphosphoric acidand perphosphoric acid, (iii) at least one oxidizing substance selectedfrom the group consisting of peroxide, hydrochloric chloric acid,hydrobromic bromic acid, nitric acid, and salts thereof; and (iv) atleast one substance selected from the group consisting of alkaline earthmetals, an inorganic colloid, and silane coupling agents.